In treating certain diseases such as breast cancers, it is known to introduce one or more tubes of small diameter, known as "applicator tubes" or "applicators", into the diseased organ, one or more radioactive sources being introduced into these applicators and allowed to act for a variable duration, generally on the order of one to two days. Most often, there are about 20 applicators involved, the number typically being between 5 and 30. The elements most recently used as the sources are iridium, cobalt, cesium and californium. These sources can be contained inside a tube called a "source holder", and it is this source holder that is introduced into the applicator. The sources may be in the form of grains, each of them several millimeters in length, or in the form of filaments of greater length. Hence the source holder may contain either a single filament or a plurality of grains disposed one after another, and separated from one another by spacers. The active length, that is, the length occupied by the sources, is equal to 150 mm at most. The source holder is mounted in the anterior portion of a movable cable along a guide tube that terminates in the applicator.
The problems that arise are not only that of driving the cable between an initial or storage position, in which the source holder is for instance inside a block of lead, to a final position or treatment position, in which the source holder is located inside the applicator, but also the problem of positioning the source holder, because it must be located at a very precise region inside the applicator, as well as the problem of detection of obstacles. It is essential that any obstacle encountered by the cable be detected, before it reaches its final position, and that the site of this obstacle be determined so that rapid intervention is possible.
Several solutions have been proposed so far. French Pat. No. 2 033 653 describes a device used especially in plesioradiotherapy, in which the source holder tube is driven by a metal cable. Controlling the position of the posterior end of this cable, that is, the end opposite that carrying the sources, is done with the aid of two end-of-course detectors, which detect the passage of this end. There is no provision for detection of obstacles in this device, and hence the cable is in danger of being blocked before its posterior end has passed the end-of-course detector corresponding to the final position, and thus it can happen that the sources are not located inside the applicator.
French Pat. No. 2 536 531 relates to a method of controlling the positioning of a source holder in a device of this generic type. Control is accomplished by analysis of the induced current of the motor driving the cable; the presence of an obstacle over the course causes the induced current of the motor to change, which enables the detection of the presence of this obstacle. A variation in the intensity of the current represents an increase in the load moment, and detecting this variation makes it possible to define a displacement time of the cable on which the source holder is mounted. This displacement time is compared with a reference time, which enables determining whether the cable is occupying an extreme position along its course, or is in an intermediate position. Although the presence of an obstacle can be detected with such a method, the exact position of the obstacle cannot be determined, because a certain amount of slippage, or sliding, can take place between the tube and the roller of the driving motor, and the exact length of the cable that has unwound is not known. For the same reason, it is no longer known with certainty that when the cable is in its final position, the source holder is indeed in its position inside the applicator.
European Pat. No. 0 152 124 also describes a device for driving and positioning a source holder in an applicator used in radiotherapy. In this device, the source holder is actuated via a metal cable, and the end of the cable opposite the sources is wound in a spiral over a groove provided on one face of a wheel driven by a rack. Positioning of the source holder in the applicator is done with the aid of a pneumatic system. Compressed air is sent through an opening located at one end of the guide tube, in which the cable moves, and the air exits by an opening located at the end of the guide tube closest to the applicator. The cable has a head that comes into contact with a stop when the source holder is in its place in the applicator, which prevents the air from leaving. A monitoring apparatus detects the resultant increase in pressure inside the tube sheath and stops the driving of the cable. There is no provision for an obstacle detection system in this device. As a result, if the cable is stopped by an obstacle prior to reaching its final position, the air will continue to exit via the second opening in the guide tube, and the cable will continue to unwind. It is therefore impossible to know whether there is an obstacle, or to detect the position of such an obstacle.